This invention relates to piezoelectric ultrasonic transducers for generating and detecting acoustic energy in high temperature applications.
Piezoelectric materials are convenient for implementing ultrasonic transducers for generating and detecting acoustic energy. Many piezoelectric materials, however, cannot be used in high temperature environments. Lead titanate zirconate (PZT), for example, is a commonly used ferroelectric ceramic which has desirable piezoelectric properties. However, the Curie temperature of PZT ranges from about 200.degree. C. to 350.degree. C., and PZT transducers are generally useful only up to about 150.degree. C. An example of a higher temperature application in which ultrasonic energy may be advantageously employed involves monitoring the curing of composite fiber-reinforced plastics, such as graphite reinforced PMR-15 resin. During the curing of such composite plastics, it is desirable to adjust process parameters such as temperature and pressure in response to the state of the plastic material, which requires a non-invasive technique for determining the degree of cure of the plastic while the cure is in progress. During curing, plastics undergo large changes in elastic moduli, and it has been found that changes in their ultrasonic properties such as the velocity and attenuation of sound through the plastic can provide a measure of its degree of cure.
Commercial composite plastics, such as PMR-15, are typically cured at temperatures of the order of 350.degree. C. At these temperatures, only certain piezoelectric materials are suitable. One such material is lithium niobate (LiNbO.sub.3). Lithium niobate has a Curie temperature of 1210.degree. C., which is only 40.degree. C. less than its melting point, and can readily function as a piezoelectric element at 350.degree. C. In order to fabricate an ultrasonic transducer, a piezoelectric element may be bonded onto a metal substrate or base which serves to protect the element from the environment and to couple acoustic energy to and from the element. However, the coefficients of thermal expansion of lithium niobate are quite anisotropic, i.e., vary with crystallographic direction, for the preferred crystal cuts of lithium niobate, which makes it impossible to match the thermal expansion coefficients of a lithium niobate piezoelectric element and its metal substrate. Because of differential thermal expansion, the piezoelectric element is subjected to substantial stress which can cause it to fracture. Bonding the piezoelectric element to its metal substrate in such a manner as to minimize stress on the piezoelectric element due to differential thermal expansion, while maintaining efficient acoustic coupling presents a considerable challenge and is a problem which has heretofore not been satisfactorily solved.
Accordingly, it is desirable to provide a piezoelectric ultrasonic transducer assembly which is useful at high temperatures and which is compensated for differential thermal expansions between the piezoelectric element and its substrate in order to minimize stress on the piezoelectric element, and it is to this end that the present invention is directed.